1. Field of the Invention
The present invention relates to a film resistor used in a flow measuring apparatus such as an airflow sensor, liquid flow sensor, and the like.
2. Description of the Related Art
Generally, in an internal combustion engine, the amount of intake air is one of the most important parameters for controlling the fuel injection amount, ignition timing, and the like. A flow measuring apparatus, i.e., an airflow meter, is provided for measuring the same. One of the more common prior art airflow meters is the vane-type, which is, however, disadvantageous in scale, response speed characteristics, and the like. Recently, airflow meters having temperature-dependent resistors have been developed, which are advantageous in scale, response speed characteristics, and the like.
There are two types of airflow meters having temperature-dependent resistors, i.e., the heater-type and direct-heated type. In all types, a heating means is provided.
Various materials such as single metal, alloy, and the like may be utilized for such a heating means. However, in the case of an intake air amount sensor in an internal combustion engine, the characteristics of resistance of the heating means must be linear in accordance with the temperature, to compensate for a wide temperature range. Also, the heating means is required to be physically stable, since it is exposed to high temperatures reaching hundreds of degrees imposed by backfiring, and is exposed to an atmosphere containing gasoline vapor or other fuel or lubricant vapor having a high humidity. Platinum has been used as a metal which satisfies the above-requirements. In a prior art film resistor as the heating means using platinum, platinum is formed by evaporation or sputtering on a substrate such as ceramic, silicon or the like, however, since platinum is a stable metal, the bonding strength of platinum and the underlying or overlying insulating material is poor, and accordingly, they are easily separated from each other.